1. Technical Field of the Invention
The present invention relates generally to an accumulator fuel injection system such as a common rail system for automotive diesel engines which is designed to spray jets of high-pressure fuel into cylinders of the engine through fuel injectors, and more particularly, to such a system designed to compensate for individual variability of fuel injectors for ensuring the stability of quantity of fuel to be injected into the engine.
2. Background Art
Typical automotive fuel injection systems equipped with solenoid-operated fuel injectors each working to inject fuel into one of cylinders of an internal combustion engine are designed to calculate the time required actually to open each of the injectors to initiate the injection of fuel into the cylinder (also called an effective injection time) and the time for which the fuel is not sprayed actually due to a time lag in operation of the injector (also called an ineffective injection time) and determines the sum thereof as an on-duration (i.e., an injector drive pulse width) in which the solenoid of the injector is to be kept excited.
Typical accumulator fuel injection systems such as common rail fuel systems for diesel engines are designed to perform multiple injections: a main injection contributing to production of engine torque and a plurality of pre-injections (also called pilot injections) in which a minute amount of fuel is sprayed into the engine before the main injection for the purposes of reducing mechanical noises and vibrations of the engine and improving exhaust emissions from the engine to meet recent emission regulations. Such a multi-injection mode is achieved by actuating each of the injectors to open its nozzle needle a plurality of times in every operation cycle of one of the cylinders to produce a sequence of injections of fuel into the combustion chamber of the cylinder, thereby reducing a rapid increase in the initial injection rate to minimize the mechanical noises and vibrations of the engine.
The above type of accumulator fuel injection systems have drawback in that the individual variability or aging of the injectors results in loss of the pilot injections or an undesirable increase in injected amount of fuel, thus loosing the effect of the pilot injections. Usually, when the fuel to be sprayed by the injectors during steady running conditions of the engine lies within a lower pressure range, the quantity of the fuel sprayed actually in the pilot injections (will also be referred to as a pilot injection quantity below) per unit of an on-duration of the solenoid of the injector (i.e., the sum of width of a drive pulse applied to the solenoid establishing the ineffective injection time and width of a drive pulse applied to the solenoid establishing the effective injection time) decreases. In the following discussion, the former width will be referred to as an ineffective injection pulse width or duration. The latter width will be referred to as an effective injection pulse width or duration. The drive pulse will be referred to as an injection pulse or injection command pulse signal. Alternatively, when the fuel to be sprayed by the injectors during steady running conditions of the engine lies within a higher pressure range, the pilot injection quantity increases.
A variation in the pilot injection quantity arising from the individual variability or aging of the injectors may be eliminated by learning a correction value for the width of a basic injection pulse applied to each of the injectors using injection-to-injection quantity deviation compensation which is known to be made during steady idle modes of engine operation for the purpose of minimizing vibrations of the engine caused by a difference between speeds of pistons in cylinders of the engine resulting from a variation in actual injection quantity between the cylinders. Specifically, the injection-to-injection quantity deviation compensation is allowed to be made only when the fuel is being sprayed at lower pressures during the steady idling of the engine using the difference between speeds of the pistons. It is, however, difficult to measure such a speed difference using a sensor output indicating the speed of the engine when the fuel is being sprayed at higher pressures, and the pilot injection quantity per unit of the injection pulse width is increasing at high-speed and load conditions of the engine. There is, heretofore, no way to learn the above correction value within that range. The leaning is also allowed to be made only when the fuel is being sprayed at lower pressures during the steady idling of the engine, thus resulting in a difficulty in increasing the number of learnings. This results in a difficulty in achieving a desired pilot injection quantity during an interval between the learnings, which may lead to failures of the pilot injections or an excess of the pilot injection quantity.
Japanese Patent First Publication No. 2001-152941 teaches an accumulator fuel injection system equipped with a pilot injection quantity correction controller and a vibration sensor attached to a side wall of a cylinder block of the engine. The pilot injection quantity correction controller works to monitor an output of the vibration sensor to find whether the pilot injection has been made or not. When the pilot injection is determined not to have been made, the pilot injection quantity correction controller increases the width of the injection pulse to be applied to the injector for a subsequent pilot injection to correct the pilot injection quantity, thereby ensuring the pilot injection. This system, however, encounters the drawback in that use of the vibration sensor to monitor the pilot injection requires a lot of effort to adapt the pilot injection quantity correction controller to a variety of existing accumulator fuel injection systems.